<!DOCTYPE html>
<html lang="zh" dir="ltr" class="client-nojs">
<head>
<title>std::launder</title>
<meta charset="UTF-8">
<meta name="generator" content="MediaWiki 1.21.2">
<link rel="shortcut icon" href="../../../common/favicon.ico">
<link rel="stylesheet" href="../../../common/ext.css">
<meta name="ResourceLoaderDynamicStyles" content="">
<link rel="stylesheet" href="../../../common/site_modules.css">
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_zh_:resourceloader:filter:minify-css:7:15cea3ec788a65b5187d4018eed543bf */</style>

<script src="../../../common/startup_scripts.js"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/utility/launder","wgTitle":"cpp/utility/launder","wgCurRevisionId":65276,"wgArticleId":8585,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":[],"wgBreakFrames":false,"wgPageContentLanguage":"zh","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"zh","wgMonthNames":["","1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"],"wgMonthNamesShort":["","1月","2月","3月","4月","5月","6月","7月","8月","9月","10月","11月","12月"],"wgRelevantPageName":"cpp/utility/launder","wgUserVariant":"zh","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"zh","language":"zh","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-MathJax":1,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_zh_:resourceloader:filter:minify-js:7:258d7cd6aa9aa67dee25e01fb6a9e505 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for cpp
 * CSS class: source-cpp, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp  {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }

/*]]>*/
</style><!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]--></head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_launder skin-cppreference2 action-view cpp-navbar">
        <!-- header -->
        <!-- /header -->
        <!-- content -->
<div id="cpp-content-base">
            <div id="content">
                <a id="top"></a>
                <div id="mw-js-message" style="display:none;"></div>
                                <!-- firstHeading -->
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>launder</h1>
                <!-- /firstHeading -->
                <!-- bodyContent -->
                <div id="bodyContent">
                                        <!-- tagline -->
                    <div id="siteSub">来自cppreference.com</div>
                    <!-- /tagline -->
                                        <!-- subtitle -->
                    <div id="contentSub"><span class="subpages">&lt; <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../utility.html" title="cpp/utility">utility</a></span></div>
                    <!-- /subtitle -->
                                                            <!-- bodycontent -->
                    <div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr"><div class="t-navbar" style=""><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.html" title="cpp/language">语言</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header">标准库头文件</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../freestanding.html" title="cpp/freestanding"> 自立与有宿主实现</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../named_req.html" title="cpp/named req">具名要求</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html#.E8.AF.AD.E8.A8.80.E6.94.AF.E6.8C.81" title="cpp/utility">语言支持库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concepts.html" title="cpp/concepts">概念库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../error.html" title="cpp/error">诊断库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility">工具库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string">字符串库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="https://zh.cppreference.com/w/cpp/container" title="cpp/container">容器库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator">迭代器库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../ranges.html" title="cpp/ranges"> 范围库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm">算法库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric">数值库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io">输入/输出库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale">本地化库</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex">正则表达式库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic">原子操作库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread">线程支持库</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../filesystem.html" title="cpp/filesystem">文件系统库</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../experimental.html" title="cpp/experimental">技术规范</a> </td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../utility.html" title="cpp/utility">工具库</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../types.html" title="cpp/types">类型支持</a>（基本类型、 RTTI 、类型特性）</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../memory.html" title="cpp/memory">动态内存管理</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../error.html" title="cpp/error">错误处理</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="program.html" title="cpp/utility/program">程序工具</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="variadic.html" title="cpp/utility/variadic">变参数函数</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="feature_test.html" title="cpp/utility/feature test">库特性测试宏</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../chrono.html" title="cpp/chrono">日期和时间</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="functional.html" title="cpp/utility/functional">函数对象</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="format.html" title="cpp/utility/format"> 格式化库</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="initializer_list.html" title="cpp/utility/initializer list"><span class="t-lines"><span>initializer_list</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="source_location.html" title="cpp/utility/source location"><span class="t-lines"><span>source_location</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="bitset.html" title="cpp/utility/bitset"><span class="t-lines"><span>bitset</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="hash.html" title="cpp/utility/hash"><span class="t-lines"><span>hash</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="integer_sequence.html" title="cpp/utility/integer sequence"><span class="t-lines"><span>integer_sequence</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 关系运算符 <span class="t-mark-rev t-deprecated-cxx20">(C++20 中弃用)</span> </td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="rel_ops/operator_cmp.html" title="cpp/utility/rel ops/operator cmp"><span class="t-lines"><span>rel_ops::operator!=</span><span>rel_ops::operator&gt;</span><span>rel_ops::operator&lt;=</span><span>rel_ops::operator&gt;=</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 比较 <span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr><tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/three_way_comparable.html" title="cpp/utility/compare/three way comparable"><span class="t-lines"><span>three_way_comparable</span><span>three_way_comparable_with</span></span></a></div></div></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/strong_ordering.html" title="cpp/utility/compare/strong ordering"><span class="t-lines"><span>strong_ordering</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/weak_ordering.html" title="cpp/utility/compare/weak ordering"><span class="t-lines"><span>weak_ordering</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/partial_ordering.html" title="cpp/utility/compare/partial ordering"><span class="t-lines"><span>partial_ordering</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/common_comparison_category.html" title="cpp/utility/compare/common comparison category"><span class="t-lines"><span>common_comparison_category</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/compare_three_way_result.html" title="cpp/utility/compare/compare three way result"><span class="t-lines"><span>compare_three_way_result</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/compare_three_way.html" title="cpp/utility/compare/compare three way"><span class="t-lines"><span>compare_three_way</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/strong_order.html" title="cpp/utility/compare/strong order"><span class="t-lines"><span>strong_order</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/weak_order.html" title="cpp/utility/compare/weak order"><span class="t-lines"><span>weak_order</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/partial_order.html" title="cpp/utility/compare/partial order"><span class="t-lines"><span>partial_order</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/compare_strong_order_fallback.html" title="cpp/utility/compare/compare strong order fallback"><span class="t-lines"><span>compare_strong_order_fallback</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/compare_weak_order_fallback.html" title="cpp/utility/compare/compare weak order fallback"><span class="t-lines"><span>compare_weak_order_fallback</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/compare_partial_order_fallback.html" title="cpp/utility/compare/compare partial order fallback"><span class="t-lines"><span>compare_partial_order_fallback</span></span></a></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="compare/named_comparison_functions.html" title="cpp/utility/compare/named comparison functions"><span class="t-lines"><span>is_eq</span><span>is_neq</span><span>is_lt</span><span>is_lteq</span><span>is_gt</span><span>is_gteq</span></span></a></div></div></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 共用词汇类型</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="pair.html" title="cpp/utility/pair"><span class="t-lines"><span>pair</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="tuple.html" title="cpp/utility/tuple"><span class="t-lines"><span>tuple</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="apply.html" title="cpp/utility/apply"><span class="t-lines"><span>apply</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="make_from_tuple.html" title="cpp/utility/make from tuple"><span class="t-lines"><span>make_from_tuple</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="optional.html" title="cpp/utility/optional"><span class="t-lines"><span>optional</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="any.html" title="cpp/utility/any"><span class="t-lines"><span>any</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="variant.html" title="cpp/utility/variant"><span class="t-lines"><span>variant</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"> <br>
</td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> swap 、 forward 与 move</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="../algorithm/swap.html" title="cpp/algorithm/swap"><span class="t-lines"><span>swap</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="exchange.html" title="cpp/utility/exchange"><span class="t-lines"><span>exchange</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="forward.html" title="cpp/utility/forward"><span class="t-lines"><span>forward</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="move.html" title="cpp/utility/move"><span class="t-lines"><span>move</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="move_if_noexcept.html" title="cpp/utility/move if noexcept"><span class="t-lines"><span>move_if_noexcept</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 初等字符串转换</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="to_chars.html" title="cpp/utility/to chars"><span class="t-lines"><span>to_chars</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="from_chars.html" title="cpp/utility/from chars"><span class="t-lines"><span>from_chars</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="chars_format.html" title="cpp/utility/chars format"><span class="t-lines"><span>chars_format</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 类型运算</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="declval.html" title="cpp/utility/declval"><span class="t-lines"><span>declval</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="as_const.html" title="cpp/utility/as const"><span class="t-lines"><span>as_const</span></span></a></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>launder</span></span></strong></div><div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code><a href="../header/new.html" title="cpp/header/new">&lt;new&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
</tbody>
<tbody class="t-dcl-rev">
<tr class="t-dcl t-since-cxx17 t-until-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> T<span class="sy2">*</span> launder<span class="br0">(</span>T<span class="sy2">*</span> p<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx17">(C++17 起)</span> <br><span class="t-mark-rev t-until-cxx20">(C++20 前)</span> </td>
</tr>
<tr class="t-dcl t-since-cxx20">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span><br>
<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> T<span class="sy2">*</span> launder<span class="br0">(</span>T<span class="sy2">*</span> p<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span> </td>
</tr>
</tbody><tbody>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>获得指向位于 <code>p</code> 所表示地址的对象的指针。
</p><p>正式而言，给定
</p>
<ul><li> 表示内存中一个字节的地址 <code>A</code> 的指针 <code>p</code>
</li><li> 一个位于地址 <code>A</code> 的对象 <code>X</code> 
</li><li> <code>X</code> 在其<a href="../language/lifetime.html" title="cpp/language/lifetime">生存期</a>内
</li><li> <code>X</code> 的类型与 <code>T</code> 相同，忽略每层的 cv 限定符
</li><li> 能通过结果触及的每个字节都能通过 p 触及（若字节在与 <code>Y</code> <a href="../language/static_cast.html#.E6.8C.87.E9.92.88.E5.8F.AF.E4.BA.92.E8.BD.AC.E6.8D.A2" title="cpp/language/static cast">指针可互转换</a>的对象 <code>Z</code> 的存储内，或在以 <code>Z</code> 为元素的立即外围数组内，则能通过指向对象 <code>Y</code> 的指针触及这些字节）
</li></ul>
<p>则 <code>std::launder(p)</code> 返回 <code>T*</code> 类型的值，它指向对象 <code>X</code> 。否则行为未定义。
</p><p>若 <code>T</code> 是函数类型或（可有 cv 限定的） <code>void</code> 则程序为病式。
</p><p>若参数的值可用于<a href="../language/constant_expression.html" title="cpp/language/constant expression">核心常量表达式</a>，则 <code>std::launder</code> 可用于核心常量表达式。
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p><code>std::launder</code> 在其参数上无效果。必须用其返回值访问对象。从而，舍弃返回值始终是错误。
</p><p><code>std::launder</code> 的典型用途包括：
</p>
<ul><li> 获得指向在同类型既存对象的存储中创建的对象的指针，这里不能<a href="../language/lifetime.html#.E5.AD.98.E5.82.A8.E7.9A.84.E9.87.8D.E7.94.A8" title="cpp/language/lifetime">重用</a>指向旧对象的指针（例如，因为任一对象为基类子对象）；
</li><li> 获得指向对象的指针，该对象由布置 <code>new</code> 从指向为该对象提供存储的对象的指针创建。
</li></ul>
<p><i>可触及性</i>限制确保不能用 <code>std::launder</code> 访问不可通过原指针访问的字节，从而干涉编译器的逃逸分析。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> x<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw4">auto</span> p <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>x<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
 
<span class="kw4">int</span> x2<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span>
<span class="kw4">auto</span> p2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>x2<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> 
<span class="co1">// 未定义行为：可通过产生的指向 x2[0] 的指针触及 x2[1] ，但不可从源触及</span>
 
<span class="kw1">struct</span> X <span class="br0">{</span> <span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span> <span class="br0">}</span> x3, x4<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 标准布局；假定无填充</span>
<span class="kw4">auto</span> p3 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>x3.<span class="me1">a</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
<span class="kw4">auto</span> p4 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>x4<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>.<span class="me1">a</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// 未定义行为：可通过产生的指向 x4[0].a 的指针（它与 x4[0] 指针间可转换）触及 x4[1] ，但不可从源触及</span>
 
<span class="kw1">struct</span> Y <span class="br0">{</span> <span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy4">;</span> <span class="kw4">double</span> y<span class="sy4">;</span> <span class="br0">}</span> x5<span class="sy4">;</span>
<span class="kw4">auto</span> p5 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span><span class="kw4">int</span><span class="br0">(</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>x5.<span class="me1">a</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> 
<span class="co1">// 未定义行为：可通过产生的指向 x5.a 的指针触及 x5.y ，但不可从源触及</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;new&gt;</span>
<span class="co2">#include &lt;cstddef&gt;</span>
<span class="co2">#include &lt;cassert&gt;</span>
 
<span class="kw1">struct</span> X <span class="br0">{</span>
  <span class="kw4">const</span> <span class="kw4">int</span> n<span class="sy4">;</span> <span class="co1">// 注意： X 拥有 const 成员</span>
  <span class="kw4">int</span> m<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Y <span class="br0">{</span>
  <span class="kw4">int</span> z<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> A <span class="br0">{</span> 
    <span class="kw1">virtual</span> <span class="kw4">int</span> transmogrify<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> B <span class="sy4">:</span> A <span class="br0">{</span>
    <span class="kw4">int</span> transmogrify<span class="br0">(</span><span class="br0">)</span> override <span class="br0">{</span> new<span class="br0">(</span>this<span class="br0">)</span> A<span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> A<span class="sy4">::</span><span class="me2">transmogrify</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> new<span class="br0">(</span>this<span class="br0">)</span> B<span class="sy4">;</span> <span class="kw1">return</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span>
 
static_assert<span class="br0">(</span>sizeof<span class="br0">(</span>B<span class="br0">)</span> <span class="sy1">==</span> sizeof<span class="br0">(</span>A<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
  X <span class="sy2">*</span>p <span class="sy1">=</span> new X<span class="br0">{</span><span class="nu0">3</span>, <span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
  <span class="kw4">const</span> <span class="kw4">int</span> a <span class="sy1">=</span> p<span class="sy2">-</span><span class="sy1">&gt;</span>n<span class="sy4">;</span>
  X<span class="sy2">*</span> np <span class="sy1">=</span> new <span class="br0">(</span>p<span class="br0">)</span> X<span class="br0">{</span><span class="nu0">5</span>, <span class="nu0">6</span><span class="br0">}</span><span class="sy4">;</span>    <span class="co1">// p 不指向新对象，因为 X::n 为 const ，而 np 指向新对象</span>
  <span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> p<span class="sy2">-</span><span class="sy1">&gt;</span>n<span class="sy4">;</span> <span class="co1">// 未定义行为</span>
  <span class="kw4">const</span> <span class="kw4">int</span> c <span class="sy1">=</span> p<span class="sy2">-</span><span class="sy1">&gt;</span>m<span class="sy4">;</span> <span class="co1">// 未定义行为（即使 m 为非 const ，也不能用 p ）</span>
  <span class="kw4">const</span> <span class="kw4">int</span> d <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span>p<span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>n<span class="sy4">;</span> <span class="co1">// OK ： std::launder(p) 指向新对象</span>
  <span class="kw4">const</span> <span class="kw4">int</span> e <span class="sy1">=</span> np<span class="sy2">-</span><span class="sy1">&gt;</span>n<span class="sy4">;</span> <span class="co1">// OK</span>
 
  alignas<span class="br0">(</span>Y<span class="br0">)</span> std<span class="sy4">::</span><span class="me2">byte</span> s<span class="br0">[</span>sizeof<span class="br0">(</span>Y<span class="br0">)</span><span class="br0">]</span><span class="sy4">;</span>
  Y<span class="sy2">*</span> q <span class="sy1">=</span> new<span class="br0">(</span><span class="sy3">&amp;</span>s<span class="br0">)</span> Y<span class="br0">{</span><span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
  <span class="kw4">const</span> <span class="kw4">int</span> f <span class="sy1">=</span> <span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span>Y<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>s<span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>z<span class="sy4">;</span> <span class="co1">// 类成员访问为未定义行为：</span>
                                             <span class="co1">// reinterpret_cast&lt;Y*&gt;(&amp;s) 拥有值“指向 s 的指针”</span>
                                             <span class="co1">// 而非指向 Y 对象</span>
  <span class="kw4">const</span> <span class="kw4">int</span> g <span class="sy1">=</span> q<span class="sy2">-</span><span class="sy1">&gt;</span>z<span class="sy4">;</span> <span class="co1">// OK</span>
  <span class="kw4">const</span> <span class="kw4">int</span> h <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1">&lt;</span>Y<span class="sy2">*</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="sy3">&amp;</span>s<span class="br0">)</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>z<span class="sy4">;</span> <span class="co1">// OK</span>
 
  A i<span class="sy4">;</span>
  <span class="kw4">int</span> n <span class="sy1">=</span> i.<span class="me1">transmogrify</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
  <span class="co1">// int m = i.transmogrify(); // 未定义行为</span>
  <span class="kw4">int</span> m <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">launder</span><span class="br0">(</span><span class="sy3">&amp;</span>i<span class="br0">)</span><span class="sy2">-</span><span class="sy1">&gt;</span>transmogrify<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
  <a href="../error/assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>m <span class="sy2">+</span> n <span class="sy1">==</span> <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A"> 缺陷报告 </span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2859">LWG 2859</a>
</td>
<td> C++17
</td>
<td> <i>可触及</i>的定义未考虑来自指针可互转换的对象的指针算术
</td>
<td> 已考虑
</td></tr></table>

<!-- 
NewPP limit report
Preprocessor visited node count: 3884/1000000
Preprocessor generated node count: 9162/1000000
Post‐expand include size: 106510/2097152 bytes
Template argument size: 22532/2097152 bytes
Highest expansion depth: 16/40
Expensive parser function count: 0/100
-->

<!-- Saved in parser cache with key mwiki1-mwiki_zh_:pcache:idhash:8585-0!*!0!!*!*!zh!* and timestamp 20200312001852 -->
</div>                    <!-- /bodycontent -->
                                        <!-- printfooter -->
                    <div class="printfooter">
                    来自“<a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/utility/launder&amp;oldid=65276">https://zh.cppreference.com/mwiki/index.php?title=cpp/utility/launder&amp;oldid=65276</a>”                    </div>
                    <!-- /printfooter -->
                                                            <!-- catlinks -->
                    <!-- /catlinks -->
                                                            <div class="visualClear"></div>
                    <!-- debughtml -->
                                        <!-- /debughtml -->
                </div>
                <!-- /bodyContent -->
            </div>
        </div>
        <!-- /content -->
        <!-- footer -->
        <div id="cpp-footer-base" class="noprint">
            <div id="footer">
                        <div id="cpp-navigation">
            <h5>导航</h5>
            <ul><li><a href="https://zh.cppreference.com/w/cpp/utility/launder">Online version</a></li><li>Offline version retrieved 2020-03-14 18:00.</li></ul></div>
                        <ul id="footer-info">
                                    <li id="footer-info-lastmod"> 本页面最后修改于2020年3月11日 (星期三) 17:18。</li>
                                    <li id="footer-info-viewcount">此页面已被浏览过5,026次。</li>
                            </ul>
                    </div>
        </div>
        <!-- /footer -->
        <script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../common/skin_scripts.js"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.MathJax","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../common/site_scripts.js"></script>
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 0.082 secs. -->
	</body>
<!-- Cached 20200312012653 -->
</html>